1. Field of the Invention
The present invention relates to a method of carrying out post-treatment to a sprayed coating formed for the purpose of prevent corrosion of steel materials containing iron as a primary constituent, such as carbon steel, nickel-chromium or stainless steel, used outdoors in a bridge, port facilities, a plant, a pipe, an advertising tower, a display tower, a train, and a ship, for instance. The present invention further relates to an agent used in the method.
2. Description of the Related Art
A method of anti-corrosion spraying carried out for anti-corrosion of steel used outdoors, such as a bridge, port facilities, a plant, a pipe, an advertising tower, a display tower, a train, and a ship is defined in Japanese Industrial Standard (JIS) H8300, in which, spraying zinc, aluminum and alloys thereof is standardized. Among those metals, pure zinc, pure aluminum, zinc-aluminum alloy, and aluminum-magnesium alloy are particularly recommended as a material to be sprayed.
Among the recommended metals, pure aluminum has high resistance to acid and heat, and hence, is used not only outdoors, but also for anti-corrosion of a plant, a tank and chemical facilities, and so on. However, spotted rust is often generated on a sprayed coating early after pure aluminum was sprayed, particularly in a week after pure aluminum was sprayed.
Herein, spotted rust is a phenomenon in which water and/or humidity reach an interface between a steel and a sprayed coating, and forms a cell, resulting in that iron ions generated through the steel pass through through-holes formed in the sprayed coating and reach a surface of the sprayed coating, and then, are oxidized by air, and thus, spotted red rust are generated on a surface of the sprayed coating. Spotted rust is generally generated on a sprayed coating having a self potential higher than the same of iron, such as a sprayed coating composed of NiCr steel or stainless steel. However, polarity is reversed due to various environmental conditions even in a sprayed coating composed of aluminum alloy having a self potential lower than the same of iron, with the result that spotted rust is generated on the sprayed coating. Once spotted rust is generated, a sprayed coating is accelerated to be worn out, and thus, has a shortened lifetime. Furthermore, even if a new sprayed coating is formed over an old sprayed coating on which spotted rust is generated, adhesion and anti-corrosion of the old sprayed coating are already deteriorated, and hence, it is necessary to entirely peel off the old sprayed coating and form a new sprayed coating, resulting in a big loss in a construction period and costs.
Spotted rust is often generated particularly in outdoor environment in the case that a sprayed coating is thin, it takes much time from spraying until sealing, or a sprayed coating is insufficiently sealed. Accordingly, when pure aluminum is used for forming a sprayed coating, it is necessary to design a sprayed coating to have a sufficient thickness, tighten time management to be carried out from spraying till sealing, or carry out sealing and coating to a degree more than necessary.
Aluminum-magnesium (95:5) alloy, another recommended metal, is much used for anti-corrosion of an oil excavating plant in oil fields in the North Sea, and exhibits superior anti-corrosion performance. Aluminum-magnesium alloy is recommended also in International Organization for Standardization (ISO) 2063. However, if aluminum-magnesium alloy is actually used in outdoor environments, it is soon worn out due to environmental conditions or a certain spraying process, and spotted rust is often generated on a sprayed coating early after aluminum-magnesium alloy was sprayed, particularly in a week after sprayed. Thus, a sprayed coating composed of aluminum-magnesium alloy is short in stability. Accordingly, similarly to a sprayed coating composed of pure aluminum, it is necessary to design a sprayed coating to have a sufficient thickness, tighten time management to be carried out from spraying till sealing, or carry out sealing and coating to a degree more than necessary.
For instance, various Publications have suggested a process of sealing.
Japanese Patent Application Publication No. H11(1999)-302820 has suggested a sealing process including the step of coating a material having a high viscosity or a high polymer material containing sulfate having a high melting point, as a primary constituent, to thereby fill therewith spaces formed in a sprayed coating, for the purpose of enhancing anti-corrosion in high-temperature corrosive environment.
Japanese Re-Publication No. 2005-35829 has suggested a sealing process including the steps of coating a material having a resistance against high temperature, particularly hot water and humidity, onto aluminum for forming a sprayed coating thereon, and causing aluminum exposed in spaces formed in a sprayed coating to react with moisture in how water or humidity to thereby form aluminum hydrate for sealing.
Japanese Patent Application Publication No. 2005-15835 has suggested a sealing process including the step of coating alkali silicate aqueous solution onto a sprayed coating composed of zinc, aluminum or zinc/aluminum pseudo-alloy to thereby compose insoluble complex salt.
Japanese Patent Application Publication No. 2007-291440 has suggested a sealing process including the step of coating a sealer composed of a mixture of water-soluble polymer emulsion, colloidal silica, and active silicon water repellent to thereby form a sealed coating.
Japanese Patent Application Publication No. 2009-46765 has suggested a sealing process including the steps of coating nitrate or acetate onto a surface of a sprayed coating, and sintering the nitrate or acetate by heating to thereby form metallic oxide for the purpose of enhancing heat-resistance and corrosion-resistance in high temperature environment.
The above-mentioned countermeasures to rusting and being early worn out in a sprayed coating composed of aluminum have problems as follows.
Designing a sprayed coating to have a great thickness not only causes waste of resources, but also lengthens a construction period, resulting in cost-up in a spraying construction and deterioration of competitive power. In addition, carrying out sealing and painting to a degree more than necessary not only causes waste of resources, but also causes cost-up in a construction, and further, causes much use of volatile organic compounds (VOC) having a half of a weight of paints. This is considered a material for causing the earth to warm up, and hence, exerting a harmful influence to the environment.
A sprayed coating formed for the purpose of anti-corrosion is designed to have a thickness determined in accordance with a lifetime predetermined in line with environments. In general, a sprayed coating having a greater thickness has a longer lifetime. In Japanese Industrial Standard (JIS) H8300, whereas a sprayed coating composed of zinc, for instance, pure zinc or zinc-aluminum alloy, is recommended to have a thickness of 50 micrometers or greater, a sprayed coating composed of aluminum, for instance, pure aluminum or aluminum-magnesium alloy is recommended to have a thickness of 100 micrometers or greater. This is considered because aluminum tends to be readily rusted. If it were possible to prevent rusting, a sprayed coating could have a thickness determined in accordance with a lifetime thereof. That is, if a sprayed coating had a short lifetime, the sprayed coating could have a small thickness with the result of reduction in costs of a construction.